1. Field of the Invention
The present invention relates to a display optical system for projecting light from a reflective light-modulation device and to an image projector including the display optical system.
2. Description of the Related Art
When colors are combined using a known color-combining prism and a projection lens, it is difficult to make the projected images of each color completely coincide on a projection surface, such as a screen, because of the lateral chromatic aberration of the projection lens. In general, a reflective liquid crystal projector has a long back-focus compared to a transmissive liquid crystal projector. Therefore, it is difficult to provide a lens structure suitable for a reflective liquid crystal projector having a wide-angle lens since lateral chromatic aberration of the reflective liquid crystal projector tends to become large.
Taking these problems into consideration, a display optical system capable of correcting the lateral chromatic aberration of the projection lens is known. Such a projection lens is capable of correcting the lateral chromatic aberration of the projection lens by changing the focal length or the position of a field lens disposed in each light path of each color of light. The field lens is disposed between a liquid crystal panel and a color combining system, including two dichroic mirrors and a reflective mirror (Refer to Japanese Patent Laid-Open No. 8-201756).
Furthermore, a display optical system capable of correcting the lateral chromatic aberration of the projection lens having a correcting lens interposed between a liquid display panel and a color-combining prism, which is a cross-dichroic prism, has been proposed (Refer to Japanese Patent Laid-Open No. 11-231218 and U.S. Pat. No. 6,457,829).
The inventors have proposed a technology for providing refractive power to the incident surface or an inner surface of a prism of a display optical system using a transmissive liquid crystal panel. (Refer to Japanese Patent Laid-Open No. 2002-55210, corresponding to U.S. Pat. No. 6,590,714)
However, since the color-combining system of the display optical system disclosed in Japanese Patent Laid-Open No. 8-201756 includes two dichroic mirrors and a reflective mirror, in addition to the lateral chromatic aberration caused by the projection lens, distortion of the image occurs because the color channels of the light transmitted through the dichroic mirrors are distorted due to the distortion of the surfaces of the dichroic mirrors. As a result, the size of the projected image for each color channel does not match the calculated size, and, thus, the magnification for each color channel is not corrected effectively.
The display optical system disclosed in Japanese Patent Laid-Open No. 11-231218 and U.S. Pat. No. 6,457,829 includes a cross-dichroic prism having four right-angled triangular prisms functioning as a color-combining system. Therefore, in addition to the lateral chromatic aberration of the projection lens, deformation of the projected image occurs due to the bending of the dichroic mirror caused by angular error of the prisms. Consequently, it is difficult to completely match the projected images for the three colors: red, green, and blue.
The display optical system disclosed in Japanese Patent Laid-Open No. 2002-55210 is similar to the display optical systems disclosed in the above-mentioned documents in that the lateral chromatic aberration is corrected in a transmissive liquid crystal projector having a short back-focus. However, there is no disclosure made for correcting the lateral chromatic aberration for a reflective liquid crystal projector having a long back-focus.
Accordingly, there is a need for a display optical system capable of effectively correcting the lateral chromatic aberration for a reflective image projector having a longer back-focus compared to the back-focus of a transmissive image projector.